In situ scanning tunneling microscopy (STM) was employed to study adlayer structures of benzoic acid, terephthalic acid, and pyrazine at a well-defined Pt(111) electrode in an aqueous HClO 4 solution. Although benzoic acid formed disordered structures on Pt(111), high-resolution STM imaging was able to distinguish between the aromatic ring and two oxygen atoms of each flat lying benzoic acid molecule. Terephthalic acid formed a highly ordered adlayer with c(2√3×4)rect symmetry. Two carboxylic acid functional groups at the para-position were clearly discerned. Intermolecular interactions, presumably through hydrogen bonding, played an important role in the formation of the ordered adlayer. Pyrazine molecules formed an ordered adlayer at a relatively negative potential of 0.2 V vs a reversible hydrogen electrode. It was assumed that the molecules adsorbed horizontally at 3-fold hollow sites, forming a long range ordered adlattice of (√7×√7)R19.1°. Disordered structures were found when the electrode potential was stepped from 0.2 to 0.5 V, which suggests that the molecular plane of adsorbed pyrazine is tilted with respect to the surface of Pt(111).